SC1486A_06 Datasheet, PDF (12/30 Pages) Semtech Corporation – Complete DDR1/2/3 Power Supply Controller

SC1486A
POWER MANAGEMENT
Application Information (Cont.)
addition of extra resistance between REFOUT and the
DIMMs to avoid spurious OVP at startup, which can occur
if REFOUT rises really slowly and VTT overshoots it. The
extra resistance allows REFOUT to rise faster, avoiding
this issue.
REFIN should also be filtered so that VDDQ ripple does
not appear at the REFIN pin. If a resistor divider is used
to create REFIN from VDDQ, then a 0.1µF capacitor from
REFIN to VSSA2 will provide adequate filtering.
Dropout Performance
The output voltage adjust range for continuous-
conduction operation is limited by the fixed 550ns
(maximum) minimum off-time one-shot. For best dropout
performance, use the slowest on-time setting of 200kHz.
When working with low input voltages, the duty-factor
limit must be calculated using worst-case values for on
and off times. The IC duty-factor limitation is given by:
DUTY =
t _ON(MIN)
+ t _ON(MIN) t _{OFF(MAX )}
Be sure to include inductor resistance and MOSFET on-
state voltage drops when performing worst-case dropout
duty-factor calculations.
SC1486A System DC Accuracy (VTT Controller)
Two IC parameters effect system DC accuracy, the error
comparator offset voltage, and the switching frequency
variation with line and load. The SC1486A regulates to
the REFOUT voltage not the REFIN voltage. Since DDR
specifications are written with respect to REFOUT, the
offset of the reference buffer does not create a regulation
error.
The error comparator offset does not drift significantly
with supply and temperature. Thus, the error comparator
contributes 1% or less to DC system inaccuracy.
The on pulse in the SC1486A is calculated to give a
pseudo fixed frequency. Nevertheless, some frequency
variation with line and load can be expected. This variation
changes the output ripple voltage. Because constant on
regulators regulate to the valley of the output ripple, ½
of the output ripple appears as a DC regulation error.
For example, if REFOUT=1.25V, then the valley of the
output ripple will be 1.25V. If the ripple is 20mV with VIN
= 6V, then the DC output voltage will be 1.26V. If the
ripple is 40mV with VIN = 25V, then the DC output voltage
will be 1.27V.
1486 System DC Accuracy (VDDQ Controller)
Two IC parameters affect system DC accuracy, the error
comparator threshold voltage variation and the switching
frequency variation with line and load.
The error comparator threshold does not drift significantly
with supply and temperature. Thus, the error comparator
contributes 1% or less to DC system inaccuracy.
Board components and layout also influence DC
accuracy. The use of 1% feedback resistors contribute
1%. If tighter DC accuracy is required use 0.1% feedback
resistors.
The on pulse in the SC1486A is calculated to give a
pseudo fixed frequency. Nevertheless, some frequency
variation with line and load can be expected. This variation
changes the output ripple voltage. Because constant on
regulators regulate to the valley of the output ripple, ½
of the output ripple appears as a DC regulation error.
For example, if the feedback resistors are chosen to
divide down the output by a factor of five, the valley of
the output ripple will be 2.5V. If the ripple is 50mV with
VIN = 6V, then the measured DC output will be 2.525V.
If the ripple increases to 80mV with VIN = 25V, then the
measured DC output will be 2.540V.
The output inductor value may change with current. This
will change the output ripple and thus the DC output
voltage. It will not change the frequency.
Switching frequency variation with load can be minimized
by choosing MOSFETs with lower R_DS(ON). High R_DS(ON)
MOSFETs will cause the switching frequency to increase
as the load current increases. This will reduce the ripple
and thus the DC output voltage.
DDR Supply Selection
The SC1486A can be configured so that VTT and VDDQ
are generated directly from the battery. Alternatively, the
VTT supply can be generated from the VDDQ supply. Since
the battery configuration generally yields better efficiency
and performance, the evaluation board is configured to
generate both supplies from the battery.
 2006 Semtech Corp.
12
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